NEW YORK (Reuters Health) - Radiation and chemotherapy given to young cancer patients don't seem to increase the risk that their own children will have birth defects years later, according to a new study.

Researchers said the finding was "reassuring" because some doctors have wondered if powerful drugs and radiation -- especially radiation directed near the ovaries or testicles -- might have long-term effects on egg and sperm cells' DNA.

There's already evidence the treatments can affect a growing girl's uterus, for example, in ways that can cause other pregnancy problems.

"We do know that some of the treatments the children receive can really wreak havoc with the reproductive system," said study author Lisa Signorello, from the International Epidemiology Institute in Rockville, Maryland.

"A lot of children are rendered infertile from strong treatments. We know now that depending on the types of treatment they received, they have higher rates of miscarriage (and) higher rates of low birthweight," she told Reuters Health.

"As a cancer survivor, the worries extend to the health of your children in many ways."

For the new study, the researchers followed close to 2,800 childhood cancer survivors in the United States and Canada and gave them regular surveys into adulthood that included questions about pregnancies and births.

Signorello and her colleagues recorded all instances of birth defects in the next generation, and compared the chance of having a baby with a birth defect to the dose of radiation or chemotherapy drugs cancer survivors had received.

Out of 4,700 babies born to survivors at least five years after they finished treatment, 129 -- just under 3 percent -- had at least one birth defect, including cleft lip and palate, Down syndrome and heart and blood vessel defects.

Babies born to parents who were treated with radiation around the testicles and ovaries or chemotherapy drugs had similar rates of birth defects as those born to parents who'd never received radiation or the DNA-altering drugs.

Among babies of women who'd survived childhood cancer, those rates were 3 percent after chemo and radiation compared to 3.5 percent when mothers hadn't had those treatments. They were 1.9 percent and 1.7 percent, respectively, in babies born to fathers who'd had cancer.

And higher doses of chemotherapy drugs or radiation to the areas around the testicles and ovaries -- such as for treatment of kidney cancer -- weren't linked to a greater chance of having a baby with a birth defect than low doses.

About 3 percent of U.S. babies have a birth defect, Signorello and her co-authors wrote Monday in the Journal of Clinical Oncology. Their results from childhood cancer survivors don't include birth defects that were definitively linked to family history, and the study did not have a control group of parents who'd never had cancer.

"These kinds of studies are very important in terms of counseling for children who have cancer and go through these treatments," said Anna Chiarelli, a senior scientist at Cancer Care Ontario in Toronto, who has studied pregnancy outcomes after childhood cancer treatment.

"There has always been that concern, what effect it will have, because of course the child is in a developing phase," she told Reuters Health.

Some 11,000 U.S. kids under age 14 will be diagnosed with cancer this year, representing less than 1 percent of new cancer diagnoses, according to the American Cancer Society.

Though previous studies also haven't found any extra birth defect risk after cancer treatment, Signorello said the size of the current study, and the fact that her team had access to medical records with information on cancer treatment, make the findings more conclusive.

Chiarelli, who wasn't involved in the new research, agreed. "This is a very significant study. I think it has gone further than the other ones," she said.

For childhood cancer survivors treated with chemo and radiation, she added, "If they choose to have children, they should feel reassured that for either (men or women) it seems their risk is no different from children who didn't have those types of therapies."

"I think on the heels of a number of studies that have tried to look at this, I hope (people) see this as a positive message and a reassuring one, and one that doctors can use to provide the best kind of information to their patients," Signorello concluded.